The Purple X is the starting point like mentioned and the starting point where things increase or decrease in size.
For your example the starting point is also locked to the top left corner. When you draw sketch on a face if your sketch end point/midpoint touches the face’s end or midpoint the sketch will automatically lock at that point.
Also if you type in a dimension it’s now locked to that length, if you hover over your dimension you will see a new locked badge which you can click to disable.